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Credit Facilities, Long-Term Debt and Lease Liabilities - Credit facilities summarized (Details) - CAD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
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Disclosure of detailed information about borrowings [line items] |  |
Facility size | $ 2,190 |
Outstanding letters of credit | 621 |
Borrowings | 3,361 | $ 3,212
Available capacity | 1,455 |
Financial assets pledged as collateral for liabilities or contingent liabilities | 0 | 3
TransAlta Renewables Inc. |  |
Disclosure of detailed information about borrowings [line items] |  |
Financial assets pledged as collateral for liabilities or contingent liabilities | 49 |
Revolving committed syndicated bank facility |  |
Disclosure of detailed information about borrowings [line items] |  |
Facility size | 1,250 |
Outstanding letters of credit | 379 |
Borrowings | 114 |
Available capacity | 757 |
Canadian committed bilateral credit facilities |  |
Disclosure of detailed information about borrowings [line items] |  |
Facility size | 240 |
Outstanding letters of credit | 150 |
Borrowings | 0 |
Available capacity | 90 |
Committed Credit Facility | TransAlta Renewables Inc. |  |
Disclosure of detailed information about borrowings [line items] |  |
Facility size | 700 |
Outstanding letters of credit | 92 |
Borrowings | 0 |
Available capacity | 608 |
Credit facilities |  |
Disclosure of detailed information about borrowings [line items] |  |
Borrowings | 114 | $ 220
Letter of credit |  |
Disclosure of detailed information about borrowings [line items] |  |
Debt instruments issued | 89 |
Letter of credit | TransAlta Renewables Inc. |  |
Disclosure of detailed information about borrowings [line items] |  |
Debt instruments issued | 92 |
Bilateral Credit Facility | Later than six months and not later than one year |  |
Disclosure of detailed information about borrowings [line items] |  |
Borrowings | $ 80 |

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- Definition The amount of instruments issued by the entity that represent indebtedness. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 1 -IssueDate 2019-01-01 -Paragraph 55 -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2019-en-r&anchor=para_55&doctype=Standard -URIDate 2019-03-27 + Details
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- Definition The amount of financial assets that the entity has pledged as collateral for liabilities or contingent liabilities, including amounts that have been reclassified in accordance with paragraph 3.2.23(a) of IFRS 9. [Refer: Contingent liabilities [member]; Financial assets] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 7 -IssueDate 2019-01-01 -Paragraph 14 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2019-en-r&anchor=para_14_a&doctype=Standard -URIDate 2019-03-27 + Details
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- Definition Borrowings Available Capacity + References No definition available. + Details
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- Definition Facility Size + References No definition available. + Details
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